This invention relates to a system for controlling a brushless DC motor, and more particularly to a control system controlling a semiconductor commutator on the basis of the result of detection of the value of current flowing through the semiconductor commutator supplying exciting current to the DC motor, so that the motor can be smoothly started under whatever loaded condition.
FIG. 1 is a circuit diagram of one form of a prior art control system of the kind described above. Referring to FIG. 1, an AC output of a commercial AC power source 2 is rectified by a rectifier circuit 4. A positive-side arm transistor group 8 is composed of three transistors, and a negative-side arm transistor group 10 is also composed of three transistors. A positive-side arm diode group 12 is composed of three flywheel diodes, and a negative-side arm diode group 14 is also composed of three flywheel diodes. The collectors of the individual transistors constituting the arm transistor group 8 and the cathodes of the individual diodes constituting the arm diode group 12 are common-connected to the positive-side terminal of the rectifier circuit 4. The emitters of the individual transistors constituting the arm transistor group 10 are common-connected to a terminal 16, and the anodes of the diodes constituting the arm diode group 14 are common-connected to another terminal 18 which is connected to the negative-side terminal of the rectifier circuit 4. The arm transistor groups 8, 10 and the arm diode groups 12, 14 constitute a semiconductor commutator.
A brushless DC motor 20 includes three-phase stator windings 22 of, for example, Y-connection and a rotor 24 in the form of a permanent magnet having four poles. The stator windings 22 are connected to the semiconductor commutator 6. A current detector 26 is connected between the terminal 16 of the semiconductor commutator 6 and the negative-side terminal of the rectifier circuit 4 to detect current flowing from the terminal 16 of the semiconductor commutator 6. A control circuit 28 compares the output signal of the current detector 26 indicative of the detected current value with a predetermined reference value and generates a pulse signal representing the on-duty which is dependent upon the result of comparison. On the basis of the pulse signal applied from the control circuit 28, a pulse pattern generator 30 generates and distributes a pulse pattern signal to the bases of the individual transistors constituting the arm transistor groups 8 and 10.
The brushless DC motor 20 is started under control of the control system in a manner as will be described now. First, in response to the output pulse signal from the control circuit 28, the pulse pattern generator 30 applies sequentially a low-frequency commutating pulse signal to the bases of the transistors constituting the arm transistor groups 8 and 10. As a result, every two phases of the stator windings 22 are successively excited to generate a rotating magnetic field, thereby starting rotation of the rotor 24. However, in a low rotation speed range of the brushless DC motor 20, the value of generated torque is large resulting in unstable starting, because such a motor has a starting characteristic as shown in FIG. 2. To avoid such unstable starting, it is necessary to limit the exciting current supplied to the stator windings 22 in the starting stage.
Current I.sub.W supplied from the semiconductor commutator 6 to the stator windings 22 flows from the arm transistor group 8 to the current detector 26 through the stator windings 22, arm transistor group 10 and terminal 16 and flows then to the negative-side terminal of the rectifier circuit 4.
The individual transistors constituting the arm transistor group 8 are repeatedly intermittently turned on-off at a frequency higher than the frequency of rotation of the rotor 24. Therefore, when any one of the transistors in the arm transistor group 8 is in its on-state, the current I.sub.W is supplied to the stator windings 22. On the other hand, when all of the transistors in the arm transistor group 8 are in their off-state, self-induction occurring in the stator windings 22 produces current I.sub.M which flows from the stator windings 22 and returns to the stator windings 22 through the arm transistor group 10, terminal 16, current detector 26, and terminal 18.
Therefore, current I.sub.D which is the sum of the currents I.sub.W and I.sub.M, as shown in (a) of FIG. 3, flows through the current detector 26. The value of this current I.sub.D is correlated to the load of the DC motor 20, and the heavier the load, the value of this current I.sub.D is larger. Therefore, it is preferable to control the pulse pattern generator 30 by detecting the value of the current I.sub.D, shown in (a) of FIG. 3, by the current detector 26, comparing the detected current value with the predetermined reference value, shown in (b) of FIG. 3, in a comparator which may be incorporated in the control circuit 28, applying the resultant output of the comparator to a microcomputer which may be incorporated in the control circuit 28 and applying a corresponding control signal from the microcomputer to the pulse pattern generator 30, so as to control the on-duty and the operating frequency of the transistors in the arm transistor group 8. That is, the preferred manner of control is such that the higher the output of the comparator, the on-duty and/or the operating frequency of the transistors in the arm transistor group 8 are decreased, so that the semiconductor commutator 6 can supply an appropriate output current to the stator windings 22 in the starting stage, thereby preventing generation of unnecessarily large torque to ensure smooth starting of the DC motor 20. However, in order to detect the current I.sub.D correlated to the load of the DC motor 20, the semiconductor commutator 6 of special type as shown in FIG. 1 must be used. It will be seen in FIG. 1 that the terminal 18, to which the anodes of the diodes constituting the arm diode group 14 are common-connected, is provided separately from the terminal 16 to which the emitters of the transistors constituting the arm transistor group 10 are common-connected. Such a semiconductor commutator differs in structure from those mass-produced and easily commercially available in the market. Therefore, such a special semiconductor commutator must be manufactured separately from commercially available ones, resulting in a higher system cost.